Rivers are living things. They naturally move gravel every day, more gets moved when the river is in flood, and hardly anything gets moved when the river is in low flow. If you are near a river during high flow you can hear the gravel and sometimes boulders being bounced along the river bed (this sound is loudest in the Waingawa River in the Wairarapa).

What this means is that gravel is constantly going from the top of the river to the bottom of the river. Gravel in the river comes from three places. It can either come from the hills, or the river banks or the river beds.

Gravel comes from the hills after landslips driven by storms or earthquakes occur. We haven’t had many of these in recent years, so our gravel supply to the rivers in the Wairarapa is ‘drying up’. We also limit the amount of gravel that can erode from the river banks, erosion means loss of land for landowners and under the river management schemes we protect land against erosion by making the banks more resistant to its effects. Unfortunately this means the only place gravel can be found is in the river bed, that’s why the river beds in the upper sections are getting lower, and you are beginning to see the bed rock being exposed in some places.

This bed lowering is only occurring in the steeper parts of the river. Once you get to the flatter parts of the river the opposite is happening (for this project this is somewhere along the Ruamahanga between its confluences with the Waingawa and Waiohine Rivers). In this area the river doesn’t have enough energy to carry gravel anymore, and it means all that gravel being pulled out of the upper river sections is being dropped into the channel and the river bed levels are increasing.

· To protect against private land loss

We need to do something because of impacts on adjacent land and property that occur as a result of changes within the river system. We have a requirement to prevent and mitigate against soil erosion and prevent damage by floods.

The more a river bed lowers the higher the banks become and the more unstable they become. This makes them more at risk of collapse as the natural system tries to balance itself. Vice-versa the more a river bed builds up the more chance there is of the river finding a new course as its old one becomes blocked up, this leads to bank erosion as the river seeks out and creates meanders.

· Working in the rivers

· Protecting land from erosion

· Stabilizing the hill slopes against land slips

Our work in the rivers mobilizes gravel. Every time we carry out works that disturb the river bed it loosens the gravel in the river. This makes it easier for the river to pick it up and move it. Some of our methods are aimed deliberately at loosening gravel too. This is done to prevent hard areas of gravel forming a channel diversion and causing land or property loss as the river starts to find a new course. This gravel loosening has good outcomes too, it improves nesting habitats for species like banded dotterels in the river by removing habitat favored by exotic predators.

· Give the river more space

· Gravel bed envelopes

· Encourage bank or hill erosion

· Change our river techniques

· Better implementation of River Buffer techniques

The best option is to give the river more space to do its natural thing. If we can remove the pressure on the natural environment and be more accepting of erosion of land then the problem gets much smaller. We have had some of this in place for more than 20 years. We have created river management buffers through private land. In theory these are areas of land available to the river for erosion, but it has had limited acceptance meaning that these areas in some instances are available to the river and in other areas they are still being protected. It is currently a voluntary system managed by river scheme committees. At best these buffers become well managed, native species planted river margins, at worst they remain marginal grazing land.

We are also looking at agreeing an upper and lower limit for the river bed. This will guide us in terms of when we may need to consider intervening. What we do when the river reaches these levels also needs to be considered.

Other options that may help are encouraging the supply of gravel to the river, possibly by softening the banks and encouraging erosion or similar in the upper catchment. There is always a risk with this that natural events may coincide with man-made events, and as a result we push the system too far in the opposite direction to present.

We will talk about these techniques in more detail in the coming weeks. If you have ideas please comment below.